1. Field of Art
This disclosure relates to an acoustic signal receiving apparatus in which a Fabry-Perot interferometer is used.
2. Description of the Related Art
Examples of known techniques of imaging an interior of a biological body include a photo-acoustic tomography (hereinafter, referred to as PAT).
The photo-acoustic tomography is configured to irradiate an object to be examined with pulsed light generated from a light source and detect acoustic waves, that are generated when energy of the pulsed light is absorbed in a biological body tissue (hereinafter, referred to as a photo-acoustic wave), at a plurality of spatial positions.
With photo-acoustic tomographic technology, signals of the acoustic waves are analyzed and processed and information such as distribution of optical energy absorption is visualized as a tomographic image.
As a method of receiving the acoustic waves such as the photo-acoustic waves described above (converting acoustic wave signals to electric signals), a technique that converts an acoustic wave signal once to a light intensity modulation and then converts the light intensity modulation to an electric signal is known (E. Zang, J. Laufer, and P. Beard, “Backward-mode multiwavelength photoacoustic scanner using a planer Fabry-Perot polymer film ultrasound sensor for high-resolution three-dimensional imaging of biological tissue”, Applied Optics, 47, 4. (2008)).
As a device configured to convert an acoustic wave to a light intensity signal, a structure including a relatively soft substance interposed between two parallel reflection mirrors to allow light to resonate between the two reflection mirrors, which is generally referred to as a Fabry-Perot Interferometer (FPI) is employed.
Hereinafter, the above-described structure composed of the Fabry-Perot Interferometer including the relatively soft substance interposed between the two parallel reflection mirrors is referred to as a FPI sensor.
Subsequently, a mechanism of receiving the acoustic wave by the FPI sensor will be described.
When an acoustic wave enters the FPI sensor, the film thickness between reflector plates changes, and a resonance wavelength of the FPI sensor changes. Since the FPI sensor has a resonator construction, a plurality of dips are generated in light reflectivity viewed from above the FPI sensor at positions having the resonance wavelength. The above-described dips are generally referred to as Fabry-Perot dips.
Here, when light (herein after, referred to as probe light) different from the light that generates acoustic waves is applied to the FPI sensor, and the wavelength of the probe light is adjusted to a wavelength near the resonance dips of the reflectivity of the FPI sensor, the resonance wavelength is changed when the acoustic wave enters and, consequently, the intensity of reflected light changes.
Subsequently, the light modulated in intensity by the FPI sensor is converted into the electric signal with a photo detector, whereby the acoustic wave signal is changed to the electric signal.
Hereinafter, an acoustic signal receiving apparatus including a system of receiving an acoustic signal on the basis of the above-described method of receiving the acoustic wave is referred to as FPI-PAT.